BROMIDE OP SILVER. Ag. Br. An insoluble salt of silver, of a yellowish colour, and thrown down whenever bromine in solution, or any soluble bromide, is added to nitrate of silver. It is soluble in concentrated solutions of nitrate of silver, and of vaiious iodides, bromides, and analogous salts, and to a very minute extent in their dilute solutions. Like other insoluble silver salts, it be,comes darker in colour in the sun's light, when nitrate of silver, or other soluble salt of that metal, is present ; and, like them, it is capable of receiv ing a latent impression by light, capable of development. When mixed with iodide of silver, it has been supposed by some to give greater sensitiveness, to increase the density of the image, and to produce a better rendering of objects which give off green rays. All these points have been disputed by others, and the opinions as to the accelerating, intensifying, and colour-rendering power of this substance are very discordant. The difference of sentiment in per sons who have tried pictures with and without it, arises from want of proper care in the conducting of the experiments, and especially from want of observation as to the purity and neutrality of the solutions employed, and as to the modifying effects of organic matter of different kinds. Nor is any mistake more common than to say one process is more sensitive than another, merely because, with the same exposure to light, that one has given a stronger or blacker colour in the shadows than the other. This difference may exist, and yet the' weaker picture may have been more completely impressed by the actinic influence ; for the true test of sensitiveness is the complete reproduction of the detail in the darkest shadows, and not the vigour of the blacks themselves. When chloride, bromide, and iodide of silver are precipitated in a pure state from nitrate of silver in excess, and exposed to light, the chloride is the first to appear discoloured, next the bromide, and lastly the iodide, and when organic matter is present, the discoloration of all is more decided, and the difference between them is still more marked than before. There is much less difference between the iodide and bromide, than between either and the chloride. When they are acted on by a developer in the presence of organic matter, it is still found that such matter has more effect in the coloration of chloride than of either of the others, and that there is still a difference between the bromide and iodide. But as to their impressibility by light, for the produc
tion of a non-apparent image, the order of these three, in similar neutral condition, is reversed, for the iodide is most quickly impressed ; though, in the process of development, the organic matter has less power of colouring and giving vigour to the image which it has received, than in the case of the bromide, and a fortiori of the chloride. In cases, therefore, where organic matter is present, to affect the result, the addition of bromide, and still more of chloride of silver, in small quantity, will give greater strength to the image ; but, at the same time, the sensitiveness is also decreased in a slight degree. But where organic matter does not interfere to a perceptible extent, as in some collodion processes, no improvement will be pro duced in the sensitiveness of the film, and but little additional inten sity given to the picture, by the use of bromides in the preparation.
The greater sensitiveness to green rays which bromide of silver was for a long time thought to enjoy, and for which it was much recommended in the photographing of foliage, is not in ordinsry practice found to exist. The coloured rays, to which iodide of silver is not sensitive, but which affect bromide, are a very incon siderable portion of those which are reflected by green objects in daylight, and scarcely affect the result. But as they exist to a greater proportional degree in artificial light of various kinds, the admixture of bromide of silver will, when they constitute the illumi nating medium, be found beneficial.
In thus recommending the use of iodide of silver alone in general operations, we speak only of average cases ; for there may occur opportunities where the properties of bromide and chloride of silver may be advantageously had recourse to ; and different kinds of organic matter introduced into the formulae, will no doubt produce corresponding modifications in the results. It is not impossible that bromide of silver may also, by its lengthening the time of exposure, sometimes usefully affect the image imprinted on the iodide with which it is combined.